Low-complexity frequency-domain nonlinear equalizer with absolute operation for underwater wireless optical communications.

A low-complexity 3-order frequency-domain nonlinear equalizer (FD-NLE) with absolute operation is proposed and experimentally demonstrated for underwater wireless optical communications (UWOC). In the proposed FD-NLE scheme, absolute operation and fast Fourier transform (FFT) with multiplication operations are utilized instead of the square and convolution operations used in conventional polynomial nonlinear equalizers (PNLEs), respectively. Therefore, complexity reductions by over 77.3% and 66.9% can be achieved compared with those of PNLE and PNLE with absolute operation, respectively, with a memory length larger than 8. A UWOC system using orthogonal frequency division multiplexing (OFDM) signals with adaptive bit and power loading is also demonstrated to evaluate the performance of the proposed scheme. Experimental results show that data rate increments by ∼ 5.6% and ∼ 5.7% at BER below 7% hard-decision forward error correction (HD-FEC) limit of 3.8 × 10, compared with PNLE and PNLE with absolute operation, respectively, are realized using the proposed scheme. Meanwhile, the proposed scheme has an up to 14.7% complexity reduction compared with conventional frequency-domain PNLE (FD-PNLE), while maintaining similar equalization performance.